Molecular sieving membranes have generated great interest as high-performance separation systems for production of clean and renewable fuels, building block chemicals, and specialty chemicals. Compared to thermodynamically-driven separation methods, membrane-based processes can significantly reduce the energy and capital costs of separating molecules on a large scale. For example, energy-intensive methods such as cryogenic distillation are commonly used to separate hydrocarbons because of their quite similar thermodynamic properties. Membranes composed of molecular sieving materials such as zeolites,1 layered zeolites,2 or metal-organic frameworks3 (MOFs) have intrinsic advantages over polymeric membranes such as a simultaneously high permeability and selectivity. Despite their performance limitations, polymeric membranes have continued to dominate industrial membrane separations due to their relative ease of processing into morphologies such as hollow fibers.4 The greatest scientific challenge facing molecular sieving membranes is the lack of an easily scalable, reliable, and benign fabrication process.5 This limitation has been particularly severe for zeolite membranes, which are typically fabricated by hydrothermal synthesis on high-cost support materials.
While substantive progress is being made in gradually reducing the barriers to economical zeolitic membranes,6-7 the advent of metal-organic framework (MOF) molecular sieves has created potential for more scalable membrane fabrication processes under relatively benign conditions.8 MOFs consist of metal centers connected by coordination bonds to organic linker molecules, and have been used to grow crystalline membranes through techniques similar to those developed for zeolitic membranes. The zeolitic imidazolite framework (ZIF) subclass of MOFs is of particular interest for membrane fabrication, because of its tunable pore size and chemistry,9 and relatively good thermal and chemical stability.10-11 In an early demonstration of scalable ZIF membrane processing, we recently demonstrated the growth of ZIF-90 membranes on the outer surfaces of porous polymeric poly(amide-imide) (Torlon®) hollow fibers (˜250 μm outer diameter) by a seeded growth process12 at mild conditions (65° C. in methanol solutions). Molecular sieving membranes on the inner surfaces of the hollow fibers are much more challenging to grow but better suited for scalable fabrication and industrial uses, due to the ability to be bundled in close proximity while avoiding membrane-membrane contact points and interfaces that lead to defects during synthesis.
It has been shown that free-standing MOF films and spheroids can be synthesized at the interfaces between two immiscible solvents.13 However, the growth of defect-controlled membranes on the inner surfaces of microscale hollow fibers (50-300 μm inner (bore) diameter) is a key, and more challenging, advance. As the bore size (and hence volume) is decreased to microscopic dimensions, the likelihood of reactant depletion and local inhomogeneity increase, leading to loss of control over membrane continuity and defect density.14 
Thus, a new reactor cell design and processing strategy for preparing defect-controlled molecular sieving membranes on the inner surfaces of microscale hollow fibers is needed to improve performance in gas and liquid separations.